Solid electrolytic capacitor containing a pre-coat layer

ABSTRACT

A solid electrolytic capacitor that contains an anode body formed from an electrically conductive powder, dielectric located over and/or within the anode body, an adhesion coating overlying the dielectric, and a solid electrolyte overlying the adhesion coating is provided. The powder has a high specific charge and in turn a relative dense packing configuration. Despite being formed from such a powder, the present inventors have discovered that the conductive polymer can be readily impregnated into the pores of the anode. This is accomplished, in part, through the use of a discontinuous precoat layer in the adhesion coating that overlies the dielectric. The precoat layer contains a plurality of discrete nanoprojections of a manganese oxide (e.g., manganese dioxide).

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/822,508 (filed on May 13, 2013) and which is incorporatedherein in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors (e.g., tantalum capacitors) have been amajor contributor to the miniaturization of electronic circuits and havemade possible the application of such circuits in extreme environments.Conventional solid electrolytic capacitors may be formed by pressing ametal powder (e.g., tantalum) around a metal lead wire, sintering thepressed part, anodizing the sintered anode, and thereafter applying asolid electrolyte. Intrinsically conductive polymers are often employedas the solid electrolyte due to their advantageous low equivalent seriesresistance (“ESR”) and “non-burning/non-ignition” failure mode. Suchelectrolytes can be formed through in situ polymerization of the monomerin the presence of a catalyst and dopant. Alternative, premadeconductive polymer slurries may also be employed. Regardless of how theyare formed, one problem with conductive polymer electrolytes is that itis difficult for such polymers to penetrate and uniformly coat the poresof the anode. Not only does this reduce the points of contact betweenthe electrolyte and dielectric, but it can also cause facilitatedelamination of the polymer from the dielectric during mounting or use.As a result of these problems, it is often difficult to achieve ultralowESR and/or leakage current values in conventional conductive polymercapacitors. The problems are compounded when the valve metal powder usedto form the anode has a high specific charge (e.g., about 70,000microFarads*Volts per gram (“μF*V/g”) or more), which is desired forachieving high capacitance values. Such high “CV/g” powders aregenerally formed from particles having a small size and large surfacearea, which results in the formation of small pores between theparticles that are even more difficult to impregnate with the conductivepolymer.

As such, a need currently exists for an improved electrolytic capacitorcontaining a conductive polymer solid electrolyte.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a solidelectrolytic capacitor is disclosed that comprises an anode body formedfrom an electrically conductive powder, wherein the powder has aspecific charge of about 70,000 μF*V/g or more. A dielectric overliesthe anode body, an adhesion coating overlies the dielectric, and solidelectrolyte overlies the dielectric. The adhesion coating includes adiscontinuous precoat layer and contains a plurality of discretenanoprojections of a manganese oxide, and the solid electrolyte includesa conductive polymer layer.

In accordance with another embodiment of the present invention, a methodfor forming a solid electrolytic capacitor is disclosed. The methodcomprises contacting an anode that contains an anode body and adielectric with a solution that contains a manganese oxide precursor,wherein the anode body is formed from an electrically conductive powderhaving a specific charge of about 70,000 μF*V/g or more. The precursoris pyrolytically converted to form a discontinuous layer containing aplurality of discrete nanoprojections of a manganese oxide. A conductivepolymer layer is formed that contacts the nanoprojections and thedielectric.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended FIGURE in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor thatmay be formed in accordance with the present invention.

Repeat use of references characters in the present specification anddrawing is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a solidelectrolytic capacitor that contains an anode body formed from anelectrically conductive powder, dielectric located over and/or withinthe anode body, an adhesion coating overlying the dielectric, and asolid electrolyte overlying the adhesion coating. The powder has a highspecific charge and in turn a relative dense packing configuration.Despite being formed from such a powder, the present inventors havediscovered that the conductive polymer can be readily impregnated intothe pores of the anode. This is accomplished, in part, through the useof a discontinuous precoat layer in the adhesion coating that overliesthe dielectric. The precoat layer contains a plurality of discretenanoprojections of a manganese oxide (e.g., manganese dioxide). Withoutintending to be limited by theory, it is believed that the small size ofthe discrete nanoprojections allows them to more readily penetrate intothe small pores of the anode body than would otherwise be possible witha conventional conductive polymer. When deposited, the nanoprojectionscan also become embedded into the conductive polymer layer as it isformed, which can enhance adhesion between the dielectric and theconductive polymer layer. The improved adhesion reduces the likelihoodof delamination and also allows a greater amount of the polymer topenetrate into the anode, thus improving capacitance while alsominimizing leakage current and ESR. Because the precoat layer is formedas discrete nanoprojections rather than as a continuous layer, theconductive polymer may also be able to contact a substantial portion ofthe dielectric, whether direction or through another layer. Therelatively large degree of direct contact between the conductive polymerand dielectric can even further reduce ESR.

Various embodiments of the invention will now be described in moredetail.

I. Anode.

As indicated above, the anode may be formed from a powder having a highspecific charge. That is, the powder may have a specific charge of about70,000 microFarads*Volts per gram (“μF*V/g”) or more, in someembodiments about 80,000 μF*V/g or more, in some embodiments about90,000 μF*V/g or more, in some embodiments from about 100,000 to about350,000 μF*V/g, and in some embodiments, from about 120,000 to about250,000 μF*V/g.

The powder may contain individual particles and/or agglomerates of suchparticles. Compounds for forming the powder include a valve metal (i.e.,metal that is capable of oxidation) or valve metal-based compound, suchas tantalum, niobium, aluminum, hafnium, titanium, alloys thereof,oxides thereof, nitrides thereof, and so forth. For example, the valvemetal composition may contain an electrically conductive oxide ofniobium, such as niobium oxide having an atomic ratio of niobium tooxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments1:1.0±0.1, and in some embodiments, 1:1.0±0.05. For example, the niobiumoxide may be NbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂. Examples of suchvalve metal oxides are described in U.S. Pat. No. 6,322,912 to Fife;U.S. Pat. No. 6,391,275 to Fife et al.; U.S. Pat. No. 6,416,730 to Fifeet al.; U.S. Pat. No. 6,527,937 to Fife; U.S. Pat. No. 6,576,099 toKimmel, et al.; U.S. Pat. No. 6,592,740 to Fife, et al.; and U.S. Pat.No. 6,639,787 to Kimmel, et al.; and U.S. Pat. No. 7,220,397 to Kimmel,al., as well as U.S. Patent Application Publication Nos. 2005/0019581 toSchnitter; 2005/0103638 to Schnitter, et al.; 2005/0013765 to Thomas, etal.

The apparent density (or Scott density) of the powder may vary asdesired, but typically ranges from about 1 to about 8 grams per cubiccentimeter (g/cm³), in some embodiments from about 2 to about 7 g/cm³,and in some embodiments, from about 3 to about 6 g/cm³. To achieve thedesired level of packing and apparent density, the size and shape of theparticles (or agglomerates) may be carefully controlled. For example,the shape of the particles may be generally spherical, nodular, etc. Theparticles may have an average size of from about 0.1 to about 20micrometers, in some embodiments from about 0.5 to about 15 micrometers,and in some embodiments, from about 1 to about 10 micrometers.

The powder may be formed using techniques known to those skilled in theart. A precursor tantalum powder, for instance, may be formed byreducing a tantalum salt (e.g., potassium fluotantalate (K₂TaF₇), sodiumfluotantalate (Na₂TaF₇), tantalum pentachloride (TaCl₅), etc.) with areducing agent (e.g., hydrogen, sodium, potassium, magnesium, calcium,etc.). Such powders may be agglomerated in a variety of ways, such asthrough one or multiple heat treatment steps at a temperature of fromabout 700° C. to about 1400° C., in some embodiments from about 750° C.to about 1200° C., and in some embodiments, from about 800° C. to about1100° C. Heat treatment may occur in an inert or reducing atmosphere.For example, heat treatment may occur in an atmosphere containinghydrogen or a hydrogen-releasing compound (e.g., ammonium chloride,calcium hydride, magnesium hydride, etc.) to partially sinter the powderand decrease the content of impurities (e.g., fluorine). If desired,agglomeration may also be performed in the presence of a gettermaterial, such as magnesium. After thermal treatment, the highlyreactive coarse agglomerates may be passivated by gradual admission ofair. Other suitable agglomeration techniques are also described in U.S.Pat. No. 6,576,038 to Rao; U.S. Pat. No. 6,238,456 to Wolf, et al.; U.S.Pat. No. 5,954,856 to Pathare, et al.; U.S. Pat. No. 5,082,491 to Rerat;U.S. Pat. No. 4,555,268 to Getz; U.S. Pat. No. 4,483,819 to Albrecht, etal.; U.S. Pat. No. 4,441,927 to Getz, et al.; and U.S. Pat. No.4,017,302 to Bates, et al.

The desired size and/or shape of the particles may be achieved bycontrolling various processing parameters as is known in the art, suchas the parameters relating to powder formation (e.g., reduction process)and/or agglomeration (e.g., temperature, atmosphere, etc.). Millingtechniques may also be employed to grind a precursor powder to thedesired size. Any of a variety of milling techniques may be utilized toachieve the desired particle characteristics. For example, the powdermay initially be dispersed in a fluid medium (e.g., ethanol, methanol,fluorinated fluid, etc.) to form a slurry. The slurry may then becombined with a grinding media (e.g., metal balls, such as tantalum) ina mill. The number of grinding media may generally vary depending on thesize of the mill, such as from about 100 to about 2000, and in someembodiments from about 600 to about 1000. The starting powder, the fluidmedium, and grinding media may be combined in any proportion. Forexample, the ratio of the starting powder to the grinding media may befrom about 1:5 to about 1:50. Likewise, the ratio of the volume of thefluid medium to the combined volume of the starting powder may be fromabout 0.5:1 to about 3:1, in some embodiments from about 0.5:1 to about2:1, and in some embodiments, from about 0.5:1 to about 1:1. Someexamples of mills that may be used in the present invention aredescribed in U.S. Pat. Nos. 5,522,558; 5,232,169; 6,126,097; and6,145,765. Milling may occur for any predetermined amount of time neededto achieve the target size. For example, the milling time may range fromabout 30 minutes to about 40 hours, in some embodiments, from about 1hour to about 20 hours, and in some embodiments, from about 5 hours toabout 15 hours. Milling may be conducted at any desired temperature,including at room temperature or an elevated temperature. After milling,the fluid medium may be separated or removed from the powder, such as byair-drying, heating, filtering, evaporating, etc.

Various other conventional treatments may also be employed in thepresent invention to improve the properties of the powder. For example,in certain embodiments, the particles may be treated with sinterretardants in the presence of a dopant, such as aqueous acids (e.g.,phosphoric acid). The amount of the dopant added depends in part on thesurface area of the powder, but is typically present in an amount of nomore than about 200 parts per million (“ppm”). The dopant may be addedprior to, during, and/or subsequent to any heat treatment step(s).

The particles may also be subjected to one or more deoxidationtreatments to improve ductility and reduce leakage current in theanodes. For example, the particles may be exposed to a getter material(e.g., magnesium), such as described in U.S. Pat. No. 4,960,471. Thegetter material may be present in an amount of from about 2% to about 6%by weight. The temperature at which deoxidation occurs may vary, buttypically ranges from about 700° C. to about 1600° C., in someembodiments from about 750° C. to about 1200° C., and in someembodiments, from about 800° C. to about 1000° C. The total time ofdeoxidation treatment(s) may range from about 20 minutes to about 3hours. Deoxidation also preferably occurs in an inert atmosphere (e.g.,argon). Upon completion of the deoxidation treatment(s), the magnesiumor other getter material typically vaporizes and forms a precipitate onthe cold wall of the furnace. To ensure removal of the getter material,however, the fine agglomerates and/or coarse agglomerates may besubjected to one or more acid leaching steps, such as with nitric acid,hydrofluoric acid, etc.

To facilitate the construction of the anode, certain components may alsobe included in the powder. For example, the powder may be optionallymixed with a binder and/or lubricant to ensure that the particlesadequately adhere to each other when pressed to form the anode body.Suitable binders may include, for instance, poly(vinyl butyral);poly(vinyl acetate); poly(vinyl alcohol); poly(vinyl pyrollidone);cellulosic polymers, such as carboxymethylcellulose, methyl cellulose,ethyl cellulose, hydroxyethyl cellulose, and methylhydroxyethylcellulose; atactic polypropylene, polyethylene; polyethylene glycol(e.g., Carbowax from Dow Chemical Co.); polystyrene,poly(butadiene/styrene); polyamides, polyimides, and polyacrylamides,high molecular weight polyethers; copolymers of ethylene oxide andpropylene oxide; fluoropolymers, such as polytetrafluoroethylene,polyvinylidene fluoride, and fluoro-olefin copolymers; acrylic polymers,such as sodium polyacrylate, poly(lower alkyl acrylates), poly(loweralkyl methacrylates) and copolymers of lower alkyl acrylates andmethacrylates; and fatty acids and waxes, such as stearic and othersoapy fatty acids, vegetable wax, microwaxes (purified paraffins), etc.The binder may be dissolved and dispersed in a solvent. Exemplarysolvents may include water, alcohols, and so forth. When utilized, thepercentage of binders and/or lubricants may vary from about 0.1% toabout 8% by weight of the total mass. It should be understood, however,that binders and/or lubricants are not necessarily required in thepresent invention.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead (e.g., tantalumwire). It should be further appreciated that the anode lead mayalternatively be attached (e.g., welded) to the anode body subsequent topressing and/or sintering of the anode body.

After compaction, any binder/lubricant may be removed by heating thepellet under vacuum at a certain temperature (e.g., from about 150° C.to about 500° C.) for several minutes. Alternatively, thebinder/lubricant may also be removed by contacting the pellet with anaqueous solution, such as described in U.S. Pat. No. 6,197,252 toBishop, et al. Thereafter, the pellet is sintered to form a porous,integral mass. For example, in one embodiment, the pellet may besintered at a temperature of from about 1200° C. to about 2000° C., andin some embodiments, from about 1500° C. to about 1800° C. under vacuumor an inert atmosphere. Upon sintering, the pellet shrinks due to thegrowth of bonds between the particles. The pressed density of the pelletafter sintering may vary, but is typically from about 2.0 to about 7.0grams per cubic centimeter, in some embodiments from about 2.5 to about6.5, and in some embodiments, from about 3.0 to about 6.0 grams percubic centimeter. The pressed density is determined by dividing theamount of material by the volume of the pressed pellet.

The anode may also have a relatively low carbon and oxygen content. Forexample, the anode may have no more than about 50 ppm carbon, and insome embodiments, no more than about 10 ppm carbon. Likewise, the anodemay have no more than about 0.15 ppm/μC/g oxygen, and in someembodiments, no more than about 0.10 ppm/μC/g oxygen. Oxygen content maybe measured by LECO Oxygen Analyzer and includes oxygen in natural oxideon the tantalum surface and bulk oxygen in the tantalum particles. Bulkoxygen content is controlled by period of crystalline lattice oftantalum, which is increasing linearly with increasing oxygen content intantalum until the solubility limit is achieved. This method wasdescribed in “Critical Oxygen Content In Porous Anodes Of Solid TantalumCapacitors”, Pozdeev-Freeman et al., Journal of Materials Science:Materials In Electronics 9, (1998) 309-311 wherein X-ray diffractionanalysis (XRDA) was employed to measure period of crystalline lattice oftantalum. Oxygen in sintered tantalum anodes may be limited to thinnatural surface oxide, while the bulk of tantalum is practically free ofoxygen.

Although not required, the thickness of the anode may be selected toimprove the electrical performance of the capacitor. For example, thethickness of the anode may be about 4 millimeters or less, in someembodiments, from about 0.05 to about 2 millimeters, and in someembodiments, from about 0.1 to about 1 millimeter. The shape of theanode may also be selected to improve the electrical properties of theresulting capacitor. For example, the anode may have a shape that iscurved, sinusoidal, rectangular, U-shaped, V-shaped, etc. The anode mayalso have a “fluted” shape in that it contains one or more furrows,grooves, depressions, or indentations to increase the surface to volumeratio to minimize ESR and extend the frequency response of thecapacitance. Such “fluted” anodes are described, for instance, in U.S.Pat. No. 6,191,936 to Webber, et al.; U.S. Pat. No. 5,949,639 to Maeda,et al.; and U.S. Pat. No. 3,345,545 to Bourgault et al., as well as U.S.Patent Application Publication No. 2005/0270725 to Hahn, et al.

II. Dielectric

The anode is also coated with a dielectric. The dielectric may be formedby anodically oxidizing (“anodizing”) the sintered anode so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude for instance, acids, such as described below with respect to theelectrolyte. For example, an acid (e.g., phosphoric acid) may constitutefrom about 0.01 wt. % to about 5 wt. %, in some embodiments from about0.05 wt. % to about 0.8 wt. %, and in some embodiments, from about 0.1wt. % to about 0.5 wt. % of the anodizing solution. If desired, blendsof acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 9 toabout 200 V, and in some embodiments, from about 20 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

If desired, each stage of anodization may be repeated for one or morecycles to achieve the desired dielectric thickness. Furthermore, theanode may also be rinsed or washed with another solvent (e.g., water)after the first and/or second stages to remove the electrolyte.

III. Adhesion Coating

As indicated above, the adhesion coating of the capacitor contains adiscontinuous precoat layer, which includes a plurality of discretenanoprojections of a manganese oxide (e.g., manganese dioxide) that canpenetrate into the small pores of the anode body and ultimately becomeembedded into the conductive polymer layer. Because the precoat layer isformed as discrete nanoprojections rather than as a continuous layer,the conductive polymer may be able to directly contact a substantialportion of the dielectric, either directly or through contact withanother layer, such as described below. The relatively large degree ofcontact between the conductive polymer and dielectric can even furtherreduce ESR. To accomplish the desired result without adversely impactingthe overall performance of the capacitor, the average size (e.g.,diameter) of the nanoprojections is typically large enough so that animprovement in adhesion is achieved, but yet not so large that they areincapable of penetrating into the pores of the anode. In this regard,the nanoprojections typically have an average size of from about 5nanometers to about 500 nanometers, in some embodiments from about 6nanometers to about 250 nanometers, in some embodiments, from about 8nanometers to about 150 nanometers, and in some embodiments, from about10 nanometers to about 110 nanometers. The term “average diameter” may,for example, refer to the average value for the major axis of thenanoprojections when viewed from above (the maximum diameter). Suchdiameters may be obtained, for example, using known techniques, such asphoton correlation spectroscopy, dynamic light scattering, quasi-elasticlight scattering, etc. Various particle size analyzers may be employedto measure the diameter in this manner. One particular example is aCorouan VASCO 3 Particle Size Analyzer. Although not necessarilyrequired, the nanoprojections may also have a narrow size distribution,which may further improve the properties of the capacitor. For instance,about 50% or more, in some embodiments about 70% or more, and in someembodiments, about 90% or more of the nanoprojections may have anaverage size within the ranges noted above. The number ofnanoprojections having a certain size may be determined using thetechniques noted above, wherein the percent volume can be correlated tothe number of particles having a certain absorbance unit (“au”).

In addition to their size, the surface coverage of the nanoprojectionson the dielectric may also be selectively controlled to help achieve thedesired electrical performance. That is, too small of a surface coveragemay limit the ability to the conductive polymer layer to better adhereto the dielectric, but too large of a coverage may adversely impact theESR of the capacitor. In this regard, the surface coverage of thenanoprojections is typically from about 0.1% to about 40%, in someembodiments from about 0.5% to about 30%, and in some embodiments, fromabout 1% to about 20%. The degree of surface coverage may be calculatedin a variety of ways, such as by dividing the “actual capacitance” valueby the “normal capacitance” value and then multiplying by 100. The“normal capacitance” is determined after forming the nanoprojections andthen impregnating the anode with the conductive polymer solution, whilethe “actual capacitance” is determined after forming thenanoprojections, impregnating the anode with the conductive polymersolution, washing the conductive polymer solution from the interior ofthe anode, and then drying the anode to remove moisture.

A variety of different techniques may be employed to form the precoatlayer of the present invention. As is known in the art, manganese oxides(e.g., manganese dioxide) are typically formed through pyrolyticdecomposition of a precursor (e.g., manganese nitrate (Mn(NO₃)₂)), suchas described in U.S. Pat. No. 4,945,452 to Sturmer, et al. For example,a dielectric-coated anode body may be contacted with a solution (e.g.,dipped, immersed, sprayed, etc.) that contains the precursor andthereafter heated for conversion into the oxide. If desired, multipleapplication steps may be employed. The amount of time in which the anodebody is in contact with a manganese oxide precursor solution may vary asdesired. For example, the anode body may be dipped into such a solutionfor a period of time ranging from about 10 seconds to about 10 minutes.

The manganese oxide precursor solution may optionally contain asurfactant. Without intending to be limited by theory, it is believedthat such a surfactant can reduce surface tension and thereby improvepenetration of the solution into the interior of the anode body.Particularly suitable are nonionic surfactants, such as a polyglycolether (e.g., polyoxyethylene alkyl ether),nonylphenoxypoly-(ethyleneoxy)ethanol (e.g., Igepal CO-630);isooctylphenoxy-polyethoxyethanol (e.g., Triton X-100),benzyletheroctylphenol-ethylene oxide condensate (e.g., Triton CF-10),3,6-dimethyl-4-octyne-3,6-diol (e.g., Surfynol 82), and so forth. Toachieve the desired improvement in the impregnation of the manganeseoxide precursor without adversely impacting other characteristics of thecapacitor, it is generally desired that the concentration of thesurfactant is selectively controlled within a certain range. Forexample, the solution into which the anode body is dipped may containthe surfactant in an amount of from about 0.01 wt. % to about 30 wt. %,in some embodiments from about 0.05 wt. % to about 25 wt. %, and in someembodiments, from about 0.1 wt. % to about 20 wt. %. The precursor(s)(e.g., manganese nitrate) may likewise constitute from about 1 wt. % toabout 55 wt. % in some embodiments from about 2 wt. % to about 15 wt. %,and in some embodiments, from about 5 wt. % to about 10 wt. %, of thesolution. A carrier, such as water, may also be employed in thesolution. Aqueous solutions of the present invention may, for instance,contain water in an amount of from about 30 wt. % to about 95 wt. %, insome embodiments from about 40 wt. % to about 99 wt. % and in someembodiments, from about 50 wt. % to about 95 wt. %. It should beunderstood that the actual amounts of the components in the solution mayvary depending upon such factors as the particle size and distributionof particles in the anode, the temperature at which decomposition isperformed, the identity of the dispersant, the identity of the carrier,etc.

If desired, the anode body may be contacted with a humidified atmospherein a pretreatment step that occurs prior to contact with a manganeseoxide precursor solution. Without intending to be limited by theory, thepresent inventors believe that the presence of a certain amount of watervapor can slow the thermal decomposition reaction of manganese dioxide,thereby causing it to form as dispersed nanoprojections. For example,during the pretreatment step, the anode body can be exposed to anatmosphere having a humidity level of from about 1 to about 30 grams ofwater per cubic meter of air (g/m³), in some embodiments from about 4 toabout 25 g/m³, and in some embodiments, from about 5 to about 20 g/m³.The relative humidity may likewise range from about 30% to about 90%, insome embodiments from about 40% to about 85%, and in some embodiments,from about 50% to about 80%. The temperature of the humidifiedatmosphere may vary, such as from about 10° C. to about 50° C., in someembodiments from about 15° C. to about 45° C., and in some embodiments,from about 20° C. to about 40° C. In addition to a pretreatment step,the anode body may also be contacted with a humidified atmosphere in anintermediate treatment step, which occurs after contact with a manganeseoxide precursor solution. The humidified atmosphere in the intermediatetreatment step may have the same or different conditions than that ofthe pretreatment step, but is generally within the ranges noted above.

Regardless, once contacted with the precursor solution for the desiredamount of time, the part is heated to a temperature sufficient topyrolytically convert the precursor (e.g., manganese nitrate) to anoxide. Heating may occur, for instance, in a furnace at a temperature offrom about 150° C. to about 300° C., in some embodiments from about 180°C. to about 290° C., and in some embodiments, from about 190° C. toabout 260° C. Heating may be conducted in a moist or dry atmosphere. Incertain embodiments, for instance, heating may be conducted in ahumidified atmosphere, which may be the same or different than theatmospheres used in the aforementioned pretreatment and intermediatetreatment steps, but generally within the conditions noted above. Thetime for the conversion depends on the furnace temperature, heattransfer rate and atmosphere, but generally is from about 3 to about 5minutes. After pyrolysis, the leakage current may sometimes be high dueto damage suffered by the dielectric film during the deposition of themanganese dioxide. To reduce this leakage, the capacitor may be reformedin an anodization bath as is known in the art. For example, thecapacitor may be dipped into an electrolyte such as described above andthen subjected to a DC current.

If desired, the adhesive coating may also contain other layers to helpreduce the likelihood of delamination. In one embodiment, for example,the adhesive coating may include a resinous layer, which may becontinuous or discontinuous in nature. When employed, the particulararrangement of the resinous layer relative to the precoat layer may varyas desired. In one embodiment, for instance, the precoat layer may beinitially formed on the dielectric, and the resinous layer maythereafter be applied to the coated dielectric. In such embodiments, theprecoat layer overlies the dielectric and the resinous layer overliesthe precoat layer and may contact the precoat layer and/or thedielectric. Despite the presence of the resinous layer, it is believedthat the coated nanoprojections of the precoat layer are still capableof becoming embedded within the conductive polymer layer. In anotherembodiment, the resinous layer may be initially applied to thedielectric, and the precoat layer may thereafter be formed thereon. Insuch embodiments, the resinous layer overlies the dielectric and theprecoat layer overlies the resinous layer.

The resinous layer may generally include a natural or synthetic resin,which may be a solid or semi-solid material that is polymeric in natureor capable of being polymerized, cured, or otherwise hardened. It isalso typically desired that the resin is relatively insulative innature. As used herein, the term “relatively insulative” generally meansmore resistive than the conductive polymer that primarily forms theconductive polymer layer. For example, in some embodiments, a relativelyinsulative resin can have a resistivity at 20° C. of about 1000 Ω-cm ormore, in some embodiments about 10,000 Ω-cm or more, in some embodimentsabout 1×10⁵ Ω-cm or more, and in some embodiments, about 1×10¹⁰ Ω-cm ormore. Some examples of suitable resins that may be employed include, butare not limited to, polyurethane, polystyrene, esters of unsaturated orsaturated fatty acids (e.g., glycerides), and so forth. For instance,suitable esters of fatty acids include, but are not limited to, estersof lauric acid, myristic acid, palmitic acid, stearic acid, eleostearicacid, oleic acid, linoleic acid, linolenic acid, aleuritic acid,shellolic acid, and so forth. These esters of fatty acids have beenfound particularly useful when used in relatively complex combinationsto form a “drying oil”, which allows the resulting film to rapidlypolymerize into a stable layer. Such drying oils may include mono-, di-,and/or tri-glycerides, which have a glycerol backbone with one, two, andthree, respectively, fatty acyl residues that are esterified. Forinstance, some suitable drying oils that may be used include, but arenot limited to, olive oil, linseed oil, castor oil, tung oil, soybeanoil, and shellac. Shellac, which is believed to contain esters ofvarious aliphatic and alicyclic hydroxy acids (e.g., aleuritic acid andshellolic acid), is particularly suitable. These and other resinmaterials are described in more detail in U.S. Pat. No. 6,674,635 toFife, et al.

When employed, the esters of fatty acids, such as described above, mayexist naturally or be refined from natural materials. For example,soybean oil is often obtained from soybeans through refinement bysolvent extraction with petroleum hydrocarbons or using continuous screwpress operations. Upon extraction, the obtained soybean oil is primarilyconstituted of triglycerides of oleic acid, linoleic acid, and linolenicacid. Tung oil, on the other hand, is a drying oil that often requiresno such refinement. In some instances, it may be desired to initiatefurther esterification of a fatty acid mixture by reacting an alcoholtherewith. Such fatty acid/alcohol ester derivatives may generally beobtained using any known alcohol capable of reacting with a fatty acid.For example, in some embodiments, monohydric and/or polyhydric alcoholswith less than 8 carbon atoms, and in some embodiments, less than 5carbon atoms, may be used in the present invention. Specific embodimentsof the present invention include the use of methanol, ethanol, butanol,as well as various glycols, such as propylene glycol, hexylene glycol,etc. In one particular embodiment, shellac can be esterified by mixingit with an alcohol, such as described above. Specifically, shellac is aresinous excretion of an insect that is believed to contain a complexmixture of fatty acids that, to some extent, are esterified. Thus, whenmixed with an alcohol, the fatty acid groups of the shellac are furtheresterified by reaction with the alcohol.

A resinous layer can be formed in a variety of different ways. Forexample, in one embodiment, the anode can be dipped into a solution ofthe desired resin(s). The solution can be formed by dissolving theselected protective resin into a solvent, such as water or a non-aqueoussolvent. Some suitable non-aqueous solvents can include, but are notlimited to, methanol, ethanol, butanol, as well as various glycols, suchas propylene glycol, hexylene glycol, di(ethylene acetate) glycol, etc.Particularly desired non-aqueous solvents are those having a boilingpoint greater than about 80° C., in some embodiments greater than about120° C., and in some embodiments, greater than about 150° C. Asdescribed above, the formation of a solution using a non-aqueous solventmay also lead to further esterification of fatty acids when suchresinous materials are utilized. The anode can be dipped into thesolution one or more times, depending on the desired thickness. Forexample, in some embodiments, multiple resinous layers may be employed,such as 2 to 10 layers, and in some embodiments, from 3 to 7 layers.Each layer may have a target thickness of, for instance, about 100nanometers or less, in some embodiments about 30 nanometers or less, andin some embodiments, about 10 nanometers or less. Besides dipping, itshould also be understood that other conventional application methods,such as sputtering, screen printing, electrophoretic coating, electronbeam deposition, vacuum deposition, spraying, and the like, can also beused.

After forming the resinous layer, the anode part may be heated orotherwise cured. Heating can facilitate evaporation of any solvent usedduring application, and may also aid in the esterification and/orpolymerization of the resinous materials. To facilitate esterificationand/or polymerization, curing agents may also be added to the resinouslayer. For instance, one example of a curing agent that can be used withshellac is sulfuric acid. The time and temperature at which heatingoccurs generally varies depending on the specific resinous materialsutilized. Typically, each layer is dried at a temperature ranging fromabout 30° C. to about 300° C., and in some embodiments, from about 50°C. to about 150° C., for a time period ranging from about 1 minute toabout 60 minutes, and in some embodiments, from about 15 minutes toabout 30 minutes. It should also be understood that heating need not beutilized after application of each resinous layer.

IV. Solid Electrolyte

As noted above, the solid electrolyte includes a conductive polymerlayer that overlies the adhesion coating and dielectric. The conductivepolymer is typically Tr-conjugated and has electrical conductivity afteroxidation or reduction, such as an electrical conductivity of at leastabout 1 μS/cm. Examples of such Tr-conjugated conductive polymersinclude, for instance, polyheterocycles (e.g., polypyrroles,polythiophenes, polyanilines, etc.), polyacetylenes, poly-p-phenylenes,polyphenolates, and so forth. In one embodiment, for example, thepolymer is a substituted polythiophene, such as those having thefollowing general structure:

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is a linear or branched, optionally substituted C₁ to C₁₈ alkylradical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- ortert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl,n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical(e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononylcyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkylradical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-,3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkylradical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0; and

n is from 2 to 5,000, in some embodiments from 4 to 2,000, and in someembodiments, from 5 to 1,000. Example of substituents for the radicals“D” or “R₇” include, for instance, alkyl, cycloalkyl, aryl, aralkyl,alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone,sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylicacid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilanegroups, carboxylamide groups, and so forth.

Particularly suitable thiophene polymers are those in which “D” is anoptionally substituted C₂ to C₃ alkylene radical. For instance, thepolymer may be optionally substituted poly(3,4-ethylenedioxythiophene),which has the following general structure:

Methods for forming conductive polymers, such as described above, arewell known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker,et al., describes various techniques for forming substitutedpolythiophenes from a monomeric precursor. The monomeric precursor may,for instance, have the following structure:

wherein,

T, D, R₇, and q are defined above. Particularly suitable thiophenemonomers are those in which “D” is an optionally substituted O₂ to O₃alkylene radical. For instance, optionally substituted3,4-alkylenedioxythiophenes may be employed that have the generalstructure:

wherein, R₇ and q are as defined above. In one particular embodiment,“q” is 0. One commercially suitable example of 3,4-ethylenedioxthiopheneis available from Heraeus Precious Metals GmbH & Co. KG under thedesignation Clevios™ M. Other suitable monomers are also described inU.S. Pat. No. 5,111,327 to Blohm, et al. and U.S. Pat. No. 6,635,729 toGroenendaal, et al. Derivatives of these monomers may also be employedthat are, for example, dimers or trimers of the above monomers. Highermolecular derivatives, i.e., tetramers, pentamers, etc. of the monomersare suitable for use in the present invention. The derivatives may bemade up of identical or different monomer units and used in pure formand in a mixture with one another and/or with the monomers. Oxidized orreduced forms of these precursors may also be employed.

The conductive polymer may be formed in situ or pre-polymerized and thenapplied to the anode body in the form of a dispersion. To form an insitu polymerized layer, the monomer may be chemically polymerized,optionally in the presence of an oxidative catalyst. The oxidativecatalyst typically includes a transition metal cation, such asiron(III), copper(II), chromium(VI), cerium(IV), manganese(IV),manganese(VII), or ruthenium(III) cations, and etc. A dopant may also beemployed to provide excess charge to the conductive polymer andstabilize the conductivity of the polymer. The dopant typically includesan inorganic or organic anion, such as an ion of a sulfonic acid. Incertain embodiments, the oxidative catalyst has both a catalytic anddoping functionality in that it includes a cation (e.g., transitionmetal) and an anion (e.g., sulfonic acid). For example, the oxidativecatalyst may be a transition metal salt that includes iron(III) cations,such as iron(III) halides (e.g., FeCl₃) or iron(III) salts of otherinorganic acids, such as Fe(ClO₄)₃ or Fe₂(SO₄)₃ and the iron(III) saltsof organic acids and inorganic acids comprising organic radicals.Examples of iron (Ill) salts of inorganic acids with organic radicalsinclude, for instance, iron(III) salts of sulfuric acid monoesters of C₁to C₂₀ alkanols (e.g., iron(III) salt of lauryl sulfate). Likewise,examples of iron(III) salts of organic acids include, for instance,iron(III) salts of C₁ to C₂₀ alkane sulfonic acids (e.g., methane,ethane, propane, butane, or dodecane sulfonic acid); iron (III) salts ofaliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid,perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron(III) salts of aliphatic C₁ to C₂₀ carboxylic acids (e.g.,2-ethylhexylcarboxylic acid); iron (III) salts of aliphaticperfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctaneacid); iron (III) salts of aromatic sulfonic acids optionallysubstituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid,o-toluene sulfonic acid, p-toluene sulfonic acid, or dodecylbenzenesulfonic acid); iron (III) salts of cycloalkane sulfonic acids (e.g.,camphor sulfonic acid); and so forth. Mixtures of these above-mentionediron(III) salts may also be used. Iron(III)-p-toluene sulfonate,iron(III)-o-toluene sulfonate, and mixtures thereof, are particularlysuitable. One commercially suitable example of iron(III)-p-toluenesulfonate is available from Heraeus Precious Metals GmbH & Co. KG underthe designation Clevios™ C.

The oxidative catalyst and monomer may be applied either sequentially ortogether to initiate the polymerization reaction. Suitable applicationtechniques for applying these components include screen-printing,dipping, electrophoretic coating, and spraying. As an example, themonomer may initially be mixed with the oxidative catalyst to form aprecursor solution. Once the mixture is formed, it may be applied to theanode part and then allowed to polymerize so that a conductive coatingis formed on the surface. Alternatively, the oxidative catalyst andmonomer may be applied sequentially. In one embodiment, for example, theoxidative catalyst is dissolved in an organic solvent (e.g., butanol)and then applied as a solution. The anode part may then be dried toremove the solvent therefrom. Thereafter, the part may be dipped into asolution containing the monomer. Regardless, polymerization is typicallyperformed at temperatures of from about −10° C. to about 250° C., and insome embodiments, from about 0° C. to about 200° C., depending on theoxidizing agent used and desired reaction time. Suitable polymerizationtechniques, such as described above, may be described in more detail inU.S. Pat. No. 7,515,396 to Biler. Still other methods for applying suchconductive coating(s) may be described in U.S. Pat. No. 5,457,862 toSakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No.5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, etal.

V. Other Layers

If desired, the capacitor may also contain other layers as is known inthe art. For example, the part may be applied with a carbon layer (e.g.,graphite) and silver layer, respectively. The silver coating may, forinstance, act as a solderable conductor, contact layer, and/or chargecollector for the capacitor and the carbon coating may limit contact ofthe silver coating with the solid electrolyte. Such coatings may coversome or all of the solid electrolyte.

VI. Terminations

The capacitor may also be provided with terminations, particularly whenemployed in surface mounting applications. For example, the capacitormay contain an anode termination to which the anode lead of thecapacitor element is electrically connected and a cathode termination towhich the cathode of the capacitor element is electrically connected.Any conductive material may be employed to form the terminations, suchas a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin,palladium, lead, copper, aluminum, molybdenum, titanium, iron,zirconium, magnesium, and alloys thereof). Particularly suitableconductive metals include, for instance, copper, copper alloys (e.g.,copper-zirconium, copper-magnesium, copper-zinc, or copper-iron),nickel, and nickel alloys (e.g., nickel-iron). The thickness of theterminations is generally selected to minimize the thickness of thecapacitor. For instance, the thickness of the terminations may rangefrom about 0.05 to about 1 millimeter, in some embodiments from about0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2millimeters. One exemplary conductive material is a copper-iron alloymetal plate available from Wieland (Germany). If desired, the surface ofthe terminations may be electroplated with nickel, silver, gold, tin,etc. as is known in the art to ensure that the final part is mountableto the circuit board. In one particular embodiment, both surfaces of theterminations are plated with nickel and silver flashes, respectively,while the mounting surface is also plated with a tin solder layer.

Referring to FIG. 1, one embodiment of an electrolytic capacitor 30 isshown that includes an anode termination 62 and a cathode termination 72in electrical connection with a capacitor element 33. The capacitorelement 33 has an upper surface 37, lower surface 39, front surface 36,and rear surface 38. Although it may be in electrical contact with anyof the surfaces of the capacitor element 33, the cathode termination 72in the illustrated embodiment is in electrical contact with the lowersurface 39 and rear surface 38. More specifically, the cathodetermination 72 contains a first component 73 positioned substantiallyperpendicular to a second component 74. The first component 73 is inelectrical contact and generally parallel with the lower surface 39 ofthe capacitor element 33. The second component 74 is in electricalcontact and generally parallel to the rear surface 38 of the capacitorelement 33. Although depicted as being integral, it should be understoodthat these portions may alternatively be separate pieces that areconnected together, either directly or via an additional conductiveelement (e.g., metal). Also, in certain embodiments, it should beunderstood that the second component 74 may be eliminated from thecathode termination 72. The anode termination 62 likewise contains afirst component 63 positioned substantially perpendicular to a secondcomponent 64. The first component 63 is in electrical contact andgenerally parallel with the lower surface 39 of the capacitor element33. The second component 64 contains a region 51 that carries an anodelead 16. In the illustrated embodiment, the region 51 possesses a“U-shape” for further enhancing surface contact and mechanical stabilityof the lead 16.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination 72 and anodetermination 62. To attach the electrolytic capacitor element 33 to thelead frame, a conductive adhesive may initially be applied to a surfaceof the cathode termination 72. The conductive adhesive may include, forinstance, conductive metal particles contained with a resin composition.The metal particles may be silver, copper, gold, platinum, nickel, zinc,bismuth, etc. The resin composition may include a thermoset resin (e.g.,epoxy resin), curing agent (e.g., acid anhydride), and coupling agent(e.g., silane coupling agents). Suitable conductive adhesives may bedescribed in U.S. Patent Publication No. 2006/0038304 to Osako, et al.Any of a variety of techniques may be used to apply the conductiveadhesive to the cathode termination 72. Printing techniques, forinstance, may be employed due to their practical and cost-savingbenefits.

A variety of methods may generally be employed to attach theterminations to the capacitor. In one embodiment, for example, thesecond component 64 of the anode termination 62 and the second component74 of the cathode termination 72 are initially bent upward to theposition shown in FIG. 1. Thereafter, the capacitor element 33 ispositioned on the cathode termination 72 so that its lower surface 39contacts the adhesive and the anode lead 16 is received by the upperU-shaped region 51. If desired, an insulating material (not shown), suchas a plastic pad or tape, may be positioned between the lower surface 39of the capacitor element 33 and the first component 63 of the anodetermination 62 to electrically isolate the anode and cathodeterminations.

The anode lead 16 is then electrically connected to the region 51 usingany technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. For example, the anode lead 16 maybe welded to the anode termination 62 using a laser. Lasers generallycontain resonators that include a laser medium capable of releasingphotons by stimulated emission and an energy source that excites theelements of the laser medium. One type of suitable laser is one in whichthe laser medium consist of an aluminum and yttrium garnet (YAG), dopedwith neodymium (Nd). The excited particles are neodymium ions Nd³⁺. Theenergy source may provide continuous energy to the laser medium to emita continuous laser beam or energy discharges to emit a pulsed laserbeam. Upon electrically connecting the anode lead 16 to the anodetermination 62, the conductive adhesive may then be cured. For example,a heat press may be used to apply heat and pressure to ensure that theelectrolytic capacitor element 33 is adequately adhered to the cathodetermination 72 by the adhesive.

Once the capacitor element is attached, the lead frame is enclosedwithin a resin casing, which may then be filled with silica or any otherknown encapsulating material. The width and length of the case may varydepending on the intended application. Suitable casings may include, forinstance, “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “I”, “J”, “K”, “L”,“M”, “N”, “P”, “R”, “S”, “T”, “V”, “W”, “X”, or “Z” cases (AVXCorporation). Regardless of the case size employed, the capacitorelement is encapsulated so that at least a portion of the anode andcathode terminations are exposed for mounting onto a circuit board. Asshown in FIG. 1, for instance, the capacitor element 33 is encapsulatedin a case 28 so that a portion of the anode termination 62 and a portionof the cathode termination 72 are exposed.

Regardless of the particular manner in which it is formed, the resultingcapacitor may exhibit excellent electrical properties. The equivalentseries resistance (“ESR”) may, for instance, be about 300 milliohms orless, in some embodiments about 200 milliohms or less, and in someembodiments, from about 1 to about 100 milliohms, as measured with a 2.2volt DC bias and a 0.5 volt peak to peak sinusoidal signal, free ofharmonics, at a frequency of 100 kHz. In addition, the leakage current,which generally refers to the current flowing from one conductor to anadjacent conductor through an insulator, can be maintained at relativelylow levels. For example, the leakage current may be about 40 μA or less,in some embodiments about 25 μA or less, and in some embodiments, about15 μA or less. The numerical value of the normalized leakage current ofthe capacitor may likewise be about 0.2 μA/μF*V or less, in someembodiments about 0.1 μA/μF*V or less, and in some embodiments, about0.05 μA/μF*V or less, where μA is microamps and μF*V is the product ofthe capacitance and the rated voltage. The ESR and normalized leakagecurrent values may even be maintained at relatively high temperatures.For example, the values may be maintained after reflow (e.g., for 10seconds) at a temperature of from about 100° C. to about 350° C., and,in some embodiments from about 200° C. to about 300° C. (e.g., 240° C.).

The present invention may be better understood with reference to thefollowing examples.

Test Procedures

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency may be 100 kHzand the temperature may be 23° C.±2° C.

Capacitance

The capacitance may be measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C.±2° C.

Leakage Current:

Leakage current (“DCL”) may be measured using a leakage test set thatmeasures leakage current at a temperature of about 25° C. and at therated voltage (e.g., 4V) after 60 seconds.

Example 1

200,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1250° C., andpressed to a density of 5.8 g/cm³. The resulting pellets had a size of0.76×1.22×0.67 mm. The pellets were anodized to 10.4V in a 0.1 wt. %nitric acid electrolyte for 10 hours to form the dielectric layer. Toform the precoat layer, the anode part was placed in a humidifiedatmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dippedfor 3 minutes in a solution containing manganese nitrate (specificgravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The partwas placed in another humidified atmosphere (30° C., humidity of 8 g/m³)for 120 minutes, and thereafter heat-treated at 250° C. in an atmospherehaving 80% relative humidity. It was determined that the resultingmanganese dioxide nanoprojections had an average size of about 10nanometers, and the surface coverage was about 10%.

Reformation was performed at 8.4V in a 0.1 wt. % acetic acid electrolytefor 20 minutes. The anode part was then dipped for 30 seconds into asolution containing 0.8 wt. % shellac and ethanol, and heat treated at125° C. for 30 minutes. To form the conductive polymer layer, the anodepart was initially dipped for 30 seconds into a solution containing 1part of 3,4-ethylenedioxthiophene monomer, 6.4 parts of an oxidizer (50wt. % iron p-toluenesulfonate), 6 parts of ethanol, and 1 part water.The monomer was allowed to polymerize for 60 minutes at 20° C. in anatmosphere containing 80% relative humidity, and then washed in asolution containing water, butanol, and p-toluenesulfonate (2 wt. %).Reformation was performed for 30 minutes at 8.4V in an electrolytecontaining 0.01 wt. % phosphoric acid. The anode part was then dippedfor 30 seconds into a solution containing an oxidizer (55 wt. % ironp-toluenesulfonate) and dried for 5 minutes at 85° C., dipped for 1second into a solution containing 3,4-ethylenedioxthiophene monomer, andthereafter allowed to polymerize for 5 to 60 minutes at 20° C. in anatmosphere containing 80% relative humidity. The part was washed in asolution containing water, butanol, and p-toluenesulfonate (2 wt. %).The part was then dipped into a graphite dispersion and dried, anddipped into a silver dispersion and dried. The finished parts werecompleted by conventional assembly technology.

After testing, it was determined that the capacitance was 45.5 μF, ESRwas 91 mΩ, and the leakage current was 14.6 μA (normalized leakagecurrent was 0.051 μA/μF*V for a rated voltage of 6.3V).

Example 2

150,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1300° C., andpressed to a density of 5.8 g/cm³. The resulting pellets had a size of1.01×1.52×0.57 mm. The pellets were anodized to 10.4V in a 0.1 wt. %nitric acid electrolyte for 10 hours to form the dielectric layer. Toform the precoat layer, the anode part was placed in a humidifiedatmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dippedfor 3 minutes in a solution containing manganese nitrate (specificgravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The partwas placed in another humidified atmosphere (30° C., humidity of 8 g/m³)for 120 minutes, and thereafter heat-treated at 250° C. in an atmospherehaving 80% relative humidity. It was determined that the resultingmanganese dioxide nanoprojections had an average size of about 11nanometers, and the surface coverage was about 10%. The remainingportions of the capacitor, including the conductive polymer layer, wereformed as described in Example 1.

After testing, it was determined that the capacitance was 46.6 μF, ESRwas 71 mΩ, and the leakage current was 12.1 μA (normalized leakagecurrent was 0.041 μA/μF*V for a rated voltage of 6.3V).

Example 3

100,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1325° C., andpressed to a density of 6.0 g/cm³. The resulting pellets had a size of0.70×1.08×0.57 mm. The pellets were anodized to 19.4V in a 0.1 wt. %nitric acid electrolyte for 8 hours to form the dielectric layer. Toform the precoat layer, the anode part was placed in a humidifiedatmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dippedfor 3 minutes in a solution containing manganese nitrate (specificgravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The partwas placed in another humidified atmosphere (30° C., humidity of 8 g/m³)for 120 minutes, and thereafter heat-treated at 250° C. in an atmospherehaving 80% relative humidity. It was determined that the resultingmanganese dioxide nanoprojections had an average size of about 11nanometers, and the surface coverage was about 10%. Reformation wasperformed at 17.4V in a 0.1 wt. % acetic acid electrolyte for 20minutes. The anode part was then dipped for 30 seconds into a solutioncontaining 0.8 wt. % shellac and ethanol, and heat treated at 125° C.for 30 minutes. The remaining portions of the capacitor, including theconductive polymer layer, were formed as described in Example 1.

After testing, it was determined that the capacitance was 9.8 μF, ESRwas 132 mΩ, and the leakage current was 0.3 μA (normalized leakagecurrent was 0.003 μA/μF*V for a rated voltage of 10V).

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A solid electrolytic capacitor comprising: ananode body formed from an electrically conductive powder, wherein thepowder has a specific charge of about 70,000 μF*V/g or more; adielectric that overlies the anode body; an adhesion coating thatoverlies the dielectric, wherein the adhesion coating includes adiscontinuous precoat layer and contains a plurality of discretenanoprojections of a manganese oxide; and a solid electrolyte thatoverlies the dielectric and includes a conductive polymer layer, whereinat least one of the discrete manganese oxide nanoprojections is embeddedin the solid electrolyte, and wherein the manganese oxidenanoprojections have an average size of from about 5 nanometers to about500 nanometers; and wherein the capacitor exhibits a normalized leakagecurrent of about 0.2 μA/μF*V or less.
 2. The solid electrolyticcapacitor of claim 1, wherein the electrically conductive powderincludes tantalum and the dielectric includes tantalum pentoxide.
 3. Thesolid electrolytic capacitor of claim 1, wherein the powder has aspecific charge of from about 120,000 μF*V/g to about 250,000 μF*V/g. 4.The solid electrolytic capacitor of claim 1, wherein about 50% or moreof the nanoprojections have an average size of from about 10 nanometersto about 110 nanometers.
 5. The solid electrolytic capacitor of claim 1,wherein the surface coverage of the nanoprojections is from about 0.1%to about 40%.
 6. The solid electrolytic capacitor of claim 1, whereinthe manganese oxide is manganese dioxide.
 7. The solid electrolyticcapacitor of claim 1, wherein the conductive polymer layer includes achemically polymerized conductive polymer.
 8. The solid electrolyticcapacitor of claim 7, wherein the chemically polymerized conductivepolymer is poly(3,4-ethylenedioxythiophene).
 9. The solid electrolyticcapacitor of claim 1, wherein the capacitor exhibits an ESR of about 300milliohms or less as determined at a frequency of 100 kHz.
 10. The solidelectrolytic capacitor of claim 1, wherein the manganese oxidenanoprojections are prepared by contacting the anode body with ahumidified atmosphere prior to contact with a solution containing amanganese oxide precursor.
 11. A method for forming a solid electrolyticcapacitor, the method comprising: contacting an anode that contains ananode body and a dielectric with a solution that contains a manganeseoxide precursor, wherein the anode body is formed from an electricallyconductive powder having a specific charge of about 70,000 μF*V/g ormore; pyrolytically converting the precursor to form a discontinuouslayer containing a plurality of discrete nanoprojections of a manganeseoxide; and forming a conductive polymer layer that contacts thenanoprojections and the dielectric, and wherein at least one of thediscrete manganese oxide nanoprojections is embedded in the solidelectrolyte, wherein the manganese oxide nanoprojections have an averagesize of from about 5 nanometers to about 500 nanometers; and wherein thecapacitor exhibits a normalized leakage current of about 02 μA/μF*V orless.
 12. The method of claim 11, wherein the manganese oxide precursoris manganese nitrate.
 13. The method of claim 11, wherein the solutioncontains a surfactant in an amount of from about 0.01 wt. % to about 30wt. %.
 14. The method of claim 11, further comprising contacting theanode with a humidified atmosphere prior to contact with the solutioncontaining the manganese oxide precursor, wherein the humidifiedatmosphere has a humidity level of from about 1 to about 30 g/m³, arelative humidity of from about 30% to about 90%, or both.
 15. Themethod of claim 11, further comprising contacting the anode with ahumidified atmosphere after contact with the solution containing themanganese oxide precursor but prior to pyrolytically converting theprecursor, wherein the humidified atmosphere has a humidity level offrom about 1 to about 30 g/m³, a relative humidity of from about 30% toabout 90%, or both.
 16. The method of claim 11, wherein the precursor ispyrolytically converted in the presence of a humidified atmospherehaving a humidity level of from about 1 to about 30 g/m³, a relativehumidity of from about 30% to about 90%, or both.
 17. The method ofclaim 11, wherein the electrically conductive powder includes tantalumand the dielectric includes tantalum pentoxide.
 18. The method of claim11, wherein the powder has a specific charge of from about 100,000μF*V/g to about 350,000 μF*V/g.
 19. The method of claim 11, wherein theconductive polymer layer is formed by chemically polymerizing athiophene monomer.